The present invention is directed to a method of operation for the fuel vapor purge system of an internal combustion engine, and more particularly to a method of operation for an electromechanical purge valve that achieves a wide range of flow control.
Effective control of evaporative emissions in a motor vehicle powered by an internal combustion engine requires a system for storing fuel tank vapor in a charcoal canister, and for activating an electromechanical purge valve to allow the stored fuel vapor to be drawn into the intake-manifold of the engine for combustion in the engine cylinders. Ordinarily, the purge valve activation level is calibrated as a function of engine operating parameters such as speed and load so that the purge vapor flow is a desired percentage of the engine airflow. The hydrocarbon concentration of the purge vapor may be estimated, and the fuel injection quantity correspondingly adjusted to maintain accurate control of the cylinder air/fuel ratio. See, for example, the co-pending U.S. patent application Ser. No. 09/264,524, filed on Mar. 8, 1999, and Ser. No. 09/950,283 filed on Sep. 10, 2001, both of which are assigned to the assignee of the present invention, and incorporated by reference herein.
Since the purge vapor flow for a given purge valve opening is limited by the intake manifold vacuum level, there are certain low-vacuum conditions under which the purge flow with a standard fully-open purge valve is too low to prevent saturation of the charcoal canister. This can occur, for example, in engines designed to operate at near-atmospheric intake manifold pressure, or in stratified combustion mode engines where the intake air flow is controlled to regulate the air/fuel ratio to a relatively high value (in this case, high throttle openings increase the intake manifold pressure). This is typically addressed by using a high-flow (i.e., large-opening) purge valve so that the desired purge flow can be achieved even at low intake manifold vacuum levels. However, using a high flow purge valve effectively raises the minimum purge flow for a given engine vacuum because the normal control range of an electromechanical valve does not include very low activation levels for which the activation level vs. valve opening relationship is highly nonlinear. As a result, the minimum flow position of a high-flow purge valve can allow higher than desired purge flow under high fuel vaporization conditions, such as when an engine is idled in a high temperature environment and/or with highly volatile fuel. Accordingly, what is needed is a control method for extending the low-flow capability of an electromechanical purge valve by utilizing its non-linear operating range.
The present invention is directed to an improved method of operation for an electromechanical purge valve of a vehicle evaporative emission control system, wherein the activation level of the purge valve is reduced below a nominal minimum level by a variable offset amount under specified operating conditions. Specifically, the low flow control is permitted when the percent of fuel from purge vapor exceeds a calibrated value and the activation level of the purge valve has been reduced to the nominal minimum, provided that the system voltage level is at or above a specified value. When low flow control is permitted, the offset amount is incrementally increased to lower the valve activation level so long as the engine fuel control is able to maintain the air/fuel ratio error at or below a calibrated amount, and incrementally decreased to raise the valve activation level when the low flow control is no longer permitted or the air/fuel ratio becomes lean enough to potentially degrade combustion stability.